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974                             Appendix V.

the boxes are lifted as soon as can be, and turned over so as to
fe* I                         empty their contents into a tank of very -cold salt water.    To

\ \                        secure bright surfaces on the work, this dumping must be done

I'}                        close to the water surface, otherwise air is admitted, and dark

| |  ,                     blue or black is the result.    It follows that pretty effects, such as

|                     streaked black or blue, may be obtained by suitable admission

of air during cooling, which may be done both by dropping the
;                      pieces from a height of one to five feet above the tank, and by

}                      allowing air from a compressing pump to flow in with the water

I                      inlet to tank.    The latter should be placed half-way up, and the

outlet should be near the bottom.    When colour is desired, the
j                      heat may be kept up for 3 or 4 hours with f" work, but must not

)                      be overdone with any, and the colour effect will depend upon the

amount of air admitted, or the height of drop.    Finally, rinse the
articles, wipe dry, finish in sawdust, and oil the surface well.

* '(                            Sometimes carbonised though soft work is required, and the

11 j                       process is called annealing.   Very old waste black is used, to

•'•"l                        which a little once-used black is added. -The heat is only applied

for a short time, and the cooling takes place very slowly, in ashesy
charcoal, or waste bone.

To avoid the necessity of re-hardening, pieces of iron rod
may be placed vertically so as to reach the centre of the box.
These are withdrawn when believed to be heated for a sufficient
time, cooled in water, and tested for depth of case or hardness.
In spite of these indicators, however, very deep carbonising may
need several heats.

The waste bone-dust is collected at the bottom of the cooling-
tank, after passing through a sieve half-way down the tank, and is
thoroughly dried in ovens before re-using.


P. 142. Cutting Speeds.—Since this text-book was first
written, a great change has occurred in the speeds allowable in
machine tools. Practice varies considerably, some users adopting
high speeds, with somewhat light cuts and fine feeds on the older
tools, while others prefer more moderate speeds with deep cuts